In a conventional system for reversing the thrust of a turbojet, the nacelle of the turbojet is provided with movable surfaces that can be deployed so as to cause a portion of the gas stream produced by the turbojet to be sent back towards the front. Deploying movable surfaces serves in particular to reduce distances required for landing. Each movable surface is deployed by means of an actuator and includes a main locking device having redundant locks for connecting the movable surface to the nacelle in order to avoid any accidental deployment of the movable surface.
Traditionally, movable surfaces are deployed by means of hydraulic actuators, which have the advantage of remaining in position so long as the pressure in the chambers of the actuator remain the same.
Proposals have been made to equip thrust reversers with electrical actuators that replace hydraulic actuators. Such an electrical actuator comprises a body and an electric motor for driving a ball screw in rotation. The ball screw comprises a screw mounted on the body to pivot and a nut engaged on the screw. The nut is provided with connection means connecting it to the movable surface and it can move from a retracted first position in which the movable surface is closed to a deployed second position in which the movable surface deflects at least a portion of the stream from the jet.
Generally, the actuators for a movable surface can also adopt an “over-retracted” third position that is reached after moving along a retraction overtravel. When the nut is in its over-retracted third position, it holds the movable surface beyond its retracted position, the movable surface then lightly compressing the gaskets of the nacelle that co-operate with the movable surface, thereby enabling the locks of the main locking device (also known as a primary lock system (PLS)) to be disengaged so as to enable subsequent deployment of the movable surface.
Nevertheless, when the motor drive of the actuator is not powered, a force applied directly to the movable surface might cause it to move (impact, vibration, air pressure under the effect of the relative wind or of the stream from the jet).
Since the consequences of undesired deployment of a movable surface of a thrust reverser are classified as being catastrophic, it is required to add an additional locking device. Proposals have been made to add an additional lock on the movable surface and also a dedicated locking-and-unlocking actuator. Since the nacelle contains a large number of mechanisms, integrating an additional lock plus actuator assembly is difficult and gives rise to extra weight. Furthermore, those elements need to be protected thermally, and they need to be designed in such a manner as to avoid generating vibration and to have a dedicated control system. All of these constraints increase the weight and the size of the equipment, thereby constituting a brake on the use of electrical actuators for thrust reverser.